Solder pastes can be supplied by printing and therefore have an advantage of high productivity. Usually, a solder paste contains a solder powder and a flux. Flux components include a reducing agent, an activator, a thixotropic agent (binder), a solvent, and a tackifier resin. The reducing agent has a role of removing oxide films on surfaces of soldering targets and the solder powder during soldering, and the activator has a role of improving reducibility and improving wettability of the solder.
The flux remains as flux residues after soldering. Among the flux components, the reducing agent and activator have a significant effect on the performance of soldering, corrosion and migration. As the reduction effect is higher, the soldering properties improve. However, these active components are easily ionized and have high affinity with water, and, therefore, tend to cause corrosion and ion migration.
For these reasons, when a solder paste containing a reducing agent and an activator is used, it is necessary to wash away flux residues after soldering as described in Patent Documents 1 and 2 (Patent Document 1, paragraphs 0002 to 0004, and Patent Document 2, paragraph 0002).